Das 1x1 der Felgenbremsen

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Dieser Artikel beschreibt die verschiedenen Arten von Felgenbremsen, deckt alle gemeinsamen Themen ab und verweist wo nötig auf spezielle Artikel, die zusätzliche Informationen liefern.

Warum Felgenbremsen?

Fahrradbremsen zerfallen in zwei generelle Kategorien: Nabenbremsen und Felgenbremsen, bei denen die Felge die zweifache Aufgabe als ungwöhnlich große Bremsscheibe erfüllt.

Die Felgenbremse funktioniert bei jeder beliebigen Laufradgröße gleich gut, weil die Geschwindigekit der Felge zur Bremse immer ungefähr gleich bleibt. Da die Felgenbremse nahezu am äußeren Rand des Laufrads wirkt, ist die Klemmkraft für eine bestimmte Bremswirkung wesentlich geringer als bei einer Nabenbremse. Durch die deutlich geringere Kraft werden Rahmen und Gabel weniger beansprucht als bei einer Nabenbremse. Felgenbremsen sind relativ sicher zur Kontrolle über die Geschwindigkeit bei langen Bergabpassagen für ein (Ein-Personen-)Fahrrad, weil die Felgen eine große Fläche für Hitzeableitung bieten.

Felgenbremsen haben allerdings auch Schwächen. Auf einem Tandem können sie die Felgen überhitzen und so zu platten Reifen führen. Ein schlecht eingestellter Bremsschuh kann den Reifen zerstören oder unter der Felge durchtauchen und mit einer Speiche das Laufrad komplett blockieren. Manche Felgenbremsarten haben Gelenke über dem Reifen und funktionieren durch den notwendigen sehr langen Hebelweg, der für dicke Reifen benötigt wird, schlechter. Nasse Felgen reduzieren die Effizienz der Bremse drastisch bei billigen Stahlfelgen und moderat bei Aluminiumfelgen.

Alles in allem sind Felgenbremsen jedoch einfach und praktisch. Man sollte ihnen den Vorzug geben, wenn Ökonomie, sportliche Leistung und Leichtbau im Vordergund stehen.

Siehe auch

• Bremsen für Tandems

Felgenbremsentypisierung

caliper-600.gif

Rim brakes fall into two basic categories.

• A caliper brake is a self-contained mechanism, attached to the bicycle's frame by a single bolt above the tire. The arms reach downward, and need to be long enough to get around the tire. The arms may pivot around the centerbolt, as in the example shown, or there may be one or more additional pivots. There is a separate article on this site identifying different caliper-brake types and pointing to information about how to care for them.
• A cantilever brake has two separate brake arms, each of which is individually attached to the frame or fork, separately on each side. Some do not look at all like the one shown, but they all have the two separately-attached arms. Cantilever brakes can only be used on bicycles that are designed or modified to use them, because they require special cantelever brakesbrazed-on fittings on the frame. These fittings are commonly called "studs" or "bosses."

There is a separate article on this site identifying the different types of cantilever brakes and pointing to further information about them.

Über Züge

Most brake problems result from excessive friction or poor installation of the cables, not poor setup, or poor quality brakes . See the article on cables for information on cable selection and adjustment of brake cables and brake levers.

Über Felgen

For a rim brake to work well, the rim must be clean and of of uniform width, and the wheel must be true. A lumpy or dented rim will make the brake grab. A rim that weaves from side to side will rub on one or both brake shoes and make you go slower whether you want to or not. A rim brake also can drag if misaligned. Cross-section of a rim with parallel braking surfaces Cross-section of a rim with angled sidewalls rim with parallel braking surfaces rim with parallel angled surfaces

Most modern rims have parallel braking surfaces. Up through the 1970s, though, most rims had angled sidewalls. Many brakes would not allow the shoes to be adjusted to the needed angle. Also, the sidewalls would act as a wedge, making the rim effectively wider where it ran closer to the hub. A rim brake would grab if there was the slightest unevenness due to a sloppy wheel truing or a curb or pothole impact. A wedge-like rim also could promote brake chatter. Some rims still are made wedge-like. They work fine with hub brakes. Avoid using them with rim brakes.

Especially in winter or in off-road riding, grit grinds away at the rim sidewalls, producing a dark-gray powder on aluminum rims that is hard to clean off hands or clothing. If the rim sidewalls are allowed to get too thin, they split. Aluminum rims have this problem bad enough. Rims with carbon-fiber braking surfaces are light, but they wear faster, require special brake shoes and are costly to replace. Many modern rims have a small pit in each sidewall to indicate a safe wear limit. When the pits disappear, the rim should be replaced.

A rim that split due to brake wear Rim which split due to brake wear

Typisierung der Bremsshuhee

A common type of brake-shoe assembly has a brake block (the rubber part that contacts the rim) which is separable from the shoe (the metal cradle for the brake block).mathauser

Older brake shoes of this type were open at one end, making it easy to slide in a replacement brake block. If the brake shoe was installed backward, the brake block could slide out in use. Modern brake shoes are idiot-proofed -- closed at both ends.

It is still often possible to pry out the brake block with a small flat-blade screwdriver, or you may have to bend down the tab at one end of the brake shoe and slide the brake block out. Bend the tab back up after installing a replacement brake block. A vise is helpful to hold the brake shoe while working with the screwdriver, and to press a brake block in or out.

Better modern brake shoes, like those shown below, have a metal plate molded into the rubber. Smooth Stud Brake shoe with smooth stud Threaded Stud (Plain Washers) Brake shoe with threaded stud Threaded Stud (Spherical Washers) Brake shoe with threaded stud

It is common for brake shoes, like the ones shown, to be longer to one side of the stud. These brake shoes are relatively shallow, allowing them to fit into the tight space between the rim and fork blade despite their greater length.

Braking force tends to rotate the brake shoes slightly in use, so the rear presses harder than the front. To equalize pressure all along the brake shoe, the longer end should face the rear of the bicycle. The result is more uniform wear and reduced tendency toward brake squeal.

There are two different ways of attaching brake shoes to the brake arms, as shown: threaded studs, and smooth studs with eye bolts.

Smooth-stud brake shoes are most often used on traditional cantilever and centerpull brakes. With modern brakes, they permit all needed types of adjustment. Older brakes such as the classic Mafacs didn't have any provision for yaw angle (toe-in) adjustment, but most cantilevers made since the 1970s have provided full adjustability.

Threaded-stud brake shoes with plain washers have limited adjustability. For this reason, they are not preferred.

Threaded-stud brake shoes with spherical (domed) washers do allow for all angle adjustments. There are two convex washers, which go inside and outside the arm, and two matching concave washers that mate with the convex ones.

Usually, the concave washers are two different thicknesses, so you can select two different extensions depending on whether you put the thick or thin concave washer on the inside.

Bremsbeläge ausrichten

Brake shoes can be adjusted in 5 different directions:

Höhe

This is the most critical adjustment. The shoe should contact the rim fully, but not overlap it. If the shoe is set too high, it will rub on the tire, destroying it very quickly. If the shoe is set too low, it can "dive" under the rim and get caught in the spokes, causing dangerous wheel lockup.

Most rim brakes have a slot in each brake arm. Height is adjusted by sliding the brake shoe post or eyebolt up or down the a slot. Different brakes have different ranges of adjustment. Sometimes, getting the height correct may require replacing a brake.

On mountain bikes with narrow rims and big bulgy tires, it may be quite difficult to get a good height adjustment with a cantilever brake that has frame or fork-mounted pivots below the rim. The shoe may clear the tire when the brake is engaged, but it may rub on the sidewall of the tire in its rest position.

With these cantilever brakes, as the brake shoe material wears down, the shoe hits lower and lower on the rim, increasing the risk of "diving" into the spokes. With other rim brakes, the shoe migrates up toward the tire. Periodic checking is in order.

Rollwinkel

The roll angle should be set so that the shoe hits the rim squarely, both the top and bottom of the shoe should meet the rim. If this is not set perfectly, normal pad wear will eventually even it out, but braking will be less effective until the shoe has worn in.

When you readjust the roll angle, you may also have to readjust the height.

Neigungswinkel

The pitch angle should be set so that the shoe follows the curvature of the rim as closely as possible. Brakes arms can flex slightly in hard use, changing the pitch angle. You may need to adjust the brake shoes to compensate, so the brake shoes don't contact the tire. Check by leaning over the bicycle and, pressing your weight down on the saddle while rolling it forward and applying each brake.

Scherwinkel ("einwärts")

The shoe should be set so that the front edge of the shoe contacts the rim slightly before the rear edge. This is commonly called "toe-in." If this is not set perfectly, normal pad wear will eventually even it out, but braking will be less effective, and the brakes may squeal until the shoe has worn in.

The traditional way to adjust yaw angle was to bend the brake arms. This can lead to failure of aluminum brake arms -- see comments by Jobst Brandt. Use brake shoes with spherical washers, or replace a smooth-stud brake that does not allow yaw-angle adjustment.

Verlängerung

Extension is the adjustment of the distance from the pad braking surface to the brake arm. Setting the shoes farther away (inward) from a cantilever arm generally increases the mechanical advantage, by increasing the cantilever angle.

The small cantilever angle of newer, narrow-profile cantilever brakes causes the anchor arm to be nearly vertical, especially on mountain bikes that have wide-set pivot bosses and narrow rims. Traditional good practice had been to slide the brake shoe holders all the way into the eyebolts, so that the back of the shoe butts up against the cantilever arm, reducing the likelihood of slippage and brake squeal. .

With the newer low-profile cantilevers, the shoe needs to be extended inward from the arm, increasing the effective cantilever angle. The unsupported length of shaft connecting the brake shoe to the arm may cause an increased tendency to squeal, but that is one of the inherent trade-offs of low-profile brakes. Increasing the extension by only a few millimeters can often dramatically improve the performance of these brakes.

With other types of rim brakes, it is usually best to keep the shoes as close to the arms as they will go, for the sake of rigidity.

Sometimes it is difficult to get all 5 adjustments set at once, and to get the brake shoe tight enough without twisting it out of position. It helps if you remove the nut and lubricate the threads, being careful not to get any lubricant on the rim or the brake shoe.

Probably the best way to adjust the shoes is to use the Shimano technique:

• Set the adjusting barrel 2-3 mm from fully closed.
• If using a cantilever brake or centerpull brake, set the link wire so that the guide line on its button lines up with the cable. If you're using a traditional yoke instead of a link wire, set the respective lengths of the main cable and transverse cable appropriately.
• With the brake in this relaxed position, adjust the brake shoes so they're touching the rims.
• Close down the adjusting barrel. This should allow the brake shoes to withdraw just enough to clear the rim.

Geräusche

Rim brakes that are working properly are generally quiet. Noises indicate problems and can also help you diagnose them.

Klappern

A chattering sound while braking may result from sticky brake-shoe deposits on the rim, or from a loose or flimsy brake. Chattering is common with cheap brakes that have thin, stamped-steel brake arms, and may require replacement of the brake. A rim with angled sidewalls promotes chattering, as the flexibility of the brake assembly allows it to ride upward toward wider part of the rim, closer to the tire -- and grab; then downward toward the hub, and loosen.

Rattern

You may hear a rattle and feel it through the handlebar when you apply the front brake lightly. The rattle stops when the brake is applied harder. The problem isn't actually in the brake at all: it is a loose headset. When you are not using the brake, your weight pushes the front fork forward. Braking pushes it backward and at some point, the forces balance and road irregularities rattle it back and forth.

A rattle that occurs when the brake is not applied and goes away when it is applied, indicates that something is loose. -- most likely, a brake pivot, or the bolt that attaches a caliper brake to the frame. Find the problem and fix it! A loose front brake that can jam in the spokes is very dangerous. A loose brake shoe can tilt and wear through the tire, or jam the wheel.

Klacken

If you bear down with your belly on the saddle and try to roll the bicycle backwards and forwards while holding a brake lever, some of the rattles described in the previous section become clunks. A clunk indicates that something is loose. Fix it!

A clunk that occurs once per wheel revolution while braking indicates that the rim has an irregular braking surface. See the longer discussion of this topic in Sheldon's article on creaks and clunks.

Schaben

Some brake shoes intended for off-road use are made of unusually hard material. It is normal for these brake shoes to make a light scraping noise in use.

Other than this, a scraping noise when the brake is applied is probably from a worn brake shoe whose metal backing plate contacts the rim. Replace the brake shoe before it wears a groove in the rim sidewall.

Musikalischer Tone, der sich mit der Geschwindigkeit verändert

A musical tone may appear when you apply the brake. The musical pitch falls as the brake slows you down. Some older (pre-1980) rims had serrated sidewalls, and with these rims, this noise is normal, if unwelcome: brakes work better on smooth rims. More often, this noise results from a brake shoe that is contacting a tire. The vibration results from the pattern of the tire fabric. The brake shoe may be misadjusted, or the wheel may be misaligned -- see Sheldon's additional comments.

Some tires have smooth rubber on their sidewalls, and then this problem doesn't make a noise. Also check the brake shoes visually. Fix this problem before the brake shoe eats a hole in the tire!

Quietschen

Squealing of brakes is a common problem, and there's no one simple solution to it.

It's caused by the friction of the brakes against the rim flexing the shoes or brake arms, which then slip back, grab, slip back, grab, etc. at high rate -- like a violin bow on a violin string. All brakes do this, but with luck the pitch (frequency) is too high for human hearing.

Occasional squeaks are annoying, but they are generally not a safety issue. On the other hand, a loud and persistent squeal makes it hard to modulate the brake. It is usually caused by a new brake shoe that has not worn in, or by grease or oil on the rim.

You might think that an oily rim would make the brake slip. Instead, it softens the brake-shoe rubber, which leaves sticky deposits on the rim. The brake shoe catches on these, squeals and grabs.

Here are some things to try if your brakes squeal (and also see Jobst Brandt's advice):

• Avoid holding the rim with greasy hands! Hold the wheel by the spokes instead.
• "Toe in" the brake shoes, so that the front edge of the shoe hits the rim slightly before the rear edge. Not all brakes permit this type of adjustment, but most do. Don't bend aluminum brake arms: this can initiate cracking and lead to failure. Instead, replace the brake if necessary.
• Clean the rims with a good, oil-free solvent (citrus, alcohol, something like that.) In obstinate cases, wash.with dishwashing detergent and a nylon-bristle brush to clean brake-shoe deposits and remaining solvent off the rim; then rinse with water. Sanding the surface of a new brake shoe lightly will remove any slippery chemical deposits. Jobst Brandt recommends using a dusting of household cleanser on a wet rim, then riding and applying the brake to clean the brake shoes and the rim.
• If the pivots of your brakes are adjustable, make sure that you've eliminated as much play as possible without causing them to bind.
• Better brake shoes may help. I particularly recommend Kool Stop salmon-colored brake shoes.
• A "brake booster" may help. This is a horseshoe-shaped arch that connects the two cantilever bolts or centerpull pivots together, making the whole system more rigid.

Zentrieren

When the brake is released, the brake shoes retract away from the rim. Ideally, the shoes on both sides should back off by the same amount. If they don't, the brake is not properly centered. In extreme cases, one of the shoes may not retract, and may rub on the rim even when the brake is not being applied.

If a brake appears off-center, check first that the wheel is installed straight in the frame/fork. If the wheel is crooked, and you maladjust the brake to compensate, you are creating two problems where there was only one before.

If the brake is, in fact, off-center, it is often the result of too much friction on one of the pivots. Unhook or loosen the cable (the transverse cable, on a cantilever or centerpull brake) and try moving each brake arm individually by hand. They should move smoothly and freely, and always come to rest near the same position. If you suspect friction, unscrew the bolt that holds arm to the boss or pivot, and remove the arm. The surface of the pivot should be smooth, free from rust, and coated with grease. If it is rusty, that is usually a sign that the bicycle was sloppily assembled by the dealer who sold it. Use emery cloth or sandpaper to remove the rust, and wipe off the dust and sand. Coat the boss or pivot with grease, and re-install the brake arm.

Rückholfedern einstellen

If your wheels are centered, and your brakes are not, and, if the pivots are properly lubricated and free-moving, the brake shoes should be centered. If they are not, you probably need to adjust the spring tension on one or both sides Different types of brakes feature different systems for adjusting the springs.

• Most caliper brakes can be centered by rotating the entire brake assembly around the bolt that attaches it to the frame or fork. Loosen the nut that secures it, and hold the bolt steady, with a wrench if possible, so that it does not rotate as you retighten the nut.

On some older and cheaper caliper brakes, including those on many English 3-speed bicycles, the return spring is rigidly attached to the spring block, which fits between the brake arms and the frame or fork. The spring block has a concave surface to mate with the convex surface of the frame or fork. It does not rotate with the brake arms. These brakes can be centered only by bending the spring or repositioning the brake shoes.

• Spring attachment. Most cantilever bosses on newer bicycles have a series of 3 small holes next to the boss itself. These holes are meant to receive and anchor the end of the return spring. Depending on which of the holes you chose to put the spring into, you have a coarse adjustment of the spring tension. The top hole provides the highest tension, the bottom hole the lowest. For most installations, the middle hole is best. (Some bosses only have one hole.) Both sides should use the same position.

Shimano cantilever

• Many Shimano style cantilevers and linear-pull brakes have a small screw on one of the cantilevers for fine balancing. This is usually on the left side of the front brake, the right side of the rear. In the early '90's, this adjustment required a 2mm Allen wrench, but newer models use a Phillips screwdriver.
• Dia Compe style cantilevers often use a totally different approach. The Dia Compe system doesn't use the spring hole in the cantilever boss, but has a separate spring block as part of the cantilever assembly. This spring block is the first part to go onto the boss, and it has a hole for the end of the spring. When the bolt holding the cantilever to the boss is loose, the spring block can turn freely, but when this bolt is tightened, it locks the spring block in place. The spring block has flats for a cone wrench (usually 13 mm) to let you rotate it to provide the desired tension.

Some Dia Compe-style brakes have these adjustable spring blocks on both sides, others have one only on one side, with the other side using the normal spring holes in the boss.

     Note: Brakes which use the adjustable spring block do not use the cantilever boss as a bearing surface; instead, they have a hollow cylindrical bushing (sleeve) which fits over the boss. The outside of this bushing/inside of the cantilever must be greased, as you would normally grease the boss.

• Non-adjustable springs are found in older or cheaper brakes. If these need adjustment, you must physically deform them. You can sometimes increase the tension of the spring by unhooking the transverse cable (or single cable of a sidepull brake) and forcing the brake arm out much farther than it would normally go. If that doesn't work, you may need to bend the spring after unhooking it from the brake, or slide a brake shoe stud in or out of the eyebolt, or reposition washers on a threaded stud.

Hebelkräfte und biegen

Mechanical advantage is a key concept, which you must understand before you will be able to choose and service brakes. Mechanical advantage is commonly referred to as "power", when discussing brakes, and it is common for people to confuse this with quality. A brake may be designed for any amount of mechanical advantage, but there is a fairly narrow range that is useable.

A few models of brake levers have adjustable mechanical advantage. Mechanical advantage may also be adjusted by changing the brake shoe extension or the length of the transverse cable if a brake uses one, or by replacing the brake, the lever or both. Pulley devices such as the Travel Agent also are available to adjust mechanical advantage.

Zu hohe Hebelkräfte

People with brake problems often think that they need more "power," when they actually need less! In particular, when modern low-profile cantilever brakes are used with drop-bar type brake levers, the combination produces excessive mechanical advantage. This problem also arises when using direct-pull cantilevers (such as Shimano's "V-Brakes") with levers made for other brakes. If there is too much mechanical advantage, the lever will be all-too-easy to pull, but it will run out of travel and bump up against the handlebar before the brake is fully applied. Once the lever hits the handlebar, it doesn't matter how much harder you squeeze! If you try to correct this by tightening up the cable, you will wind up with the brake shoes too close to the rim when at rest, which will cause them to rub, especially if the wheel isn't perfectly true.

Zu geringe Hebelkräfte

If your brakes too little mechanical advantage, when you squeeze your brake levers, you may feel a nice firm response. In fact, if you just squeeze the brakes of a bike that is not moving, your first impression may be that the brakes are in great shape, because they feel so solid and firm...the problem is, that they don't stop the bike, unless you squeeze very hard on the levers! A brake system with too little mechanical advantage will push the shoes against the rim quickly, but won't push them hard enough.

In some cases, what appears to be inadequate mechanical advantage turns out to be that the brake shoes are not "grippy" enough; they may be dirty, or dried out, or of low quality. Better-quality shoes can make a real difference. I particularly recommend Kool-Stop Salmon brake shoes, which have good grip and wear slowly.

Variable Hebelkräfte

Mechanical advantage can vary as the brake is squeezed.

Ideally, it should start out low, so that the brake shoes can be set for generous rim clearance. The low initial mechanical advantage will allow the shoes to move in toward the rim fast, in response to a small amount of hand-lever movement.

As the shoes get close to the rim, the mechanical advantage should increase, so that a small amount of hand force will result in a strong force pressing the shoes against the rim.

Some high-end brake levers, most notably the upper-end Shimano models with the "servo-wave" feature, are designed to do exactly this.

Unfortunately, conventional center-pull brakes produce exactly the opposite variation! As the lever is pulled, the yoke angle gets sharper and sharper, while, in the case of low-profile cantilever brakes, the pivot-cable distance decreases. Both of these conditions reduce the mechanical advantage as the brake is applied.

Since the travel increases as the brake shoes wear down, braking performance degrades as the shoes wear, not because the shoes have any less grip, but because the mechanical advantage has decreased.

Biegen

Preferably, every part of a rim-brake system is rigid except the brake shoes. Their compressibility allows them to conform to the shape of the rim and to take up the motion of the brake levers. If, on the other hand, the brake arms are flimsy and flexy, as on cheap long-reach sidepull caliper brakes, there is no way the brakes can develop enough "power". Most of the motion of the brake levers is wasted in flexing the arms. If the mechanical advantage is high, the brake lever will go to the handlebar before the brake shoes grip the rim very hard. If the mechanical advantage is low, the feel at the levers will be harder, but the brake will never develop enough grip.

Siehe auch

• Bremsen für Tandems

Quelle

Dieser Artikel basiert auf dem Artikel Bicycle Rim Brakes von der Website Sheldon Browns. Originalautor des Artikels ist Sheldon Brown.